Ammonia oxidation burners are used in the synthesis of nitric acid, for example. In that case ammonia (NH3) and oxygen (O2) are reacted catalytically to form nitrogen monoxide (NO) and water (H2O) in the ammonia oxidation burner. The NO obtained is then used further for preparing nitric acid.
Catalysts used in ammonia oxidation burners are customarily platinum/rhodium gauzes, which are placed on a bed consisting of particles. The particles generally are designed as packing elements made of stoneware, glass, porcelain or stainless steel, and are introduced into a burner basket arranged within the burner. The burner basket customarily has a gas-permeable bottom plate, allowing the NH3 introduced into the burner to flow through the burner basket and through the bed.
The oxidation of ammonia in the burner requires an operating temperature of about 890° C. at a pressure of about 10 bar. Owing to the high temperature, in operation of the ammonia oxidation burner, the burner basket undergoes expansion. An observation here is that the bottom plate of the burner basket expands with a delay in comparison to the side walls of the burner basket. On repeated start-up and run-down of the ammonia oxidation burner, these differences in expansion characteristics between burner basket side walls and bottom plate result in development of cavities and cracks in the bed in the edge region of the burner basket. This destruction of the bed structure reduces the flow resistance for the permeating stream of NH3, and the catalyst gauzes are no longer supported uniformly by the bed. These phenomena give rise to a loss of combustion efficiency and to ammonia slip.